AbstractStoeckly and Dressler's suggestion (Ap. J.139.240, 1964) that the diffuse band at 4430 Å might be due to absorption by Ca atoms trapped in interstellar grains has been examined in a series of laboratory experiments. It has been assumed that Ca atoms are trapped in the dielectric mantles of interstellar grains. We have studied the spectrum of Ca trapped in solid benzene, diphenyl, triphenylene, toluene, decane, tetradecane, methanol and ethanol at temperatures below 120 °K. In most systems, several absorption bands attributable to the 4226 Å resonance line of Ca are seen in the 5500 – 3800 Å region. Benzene, the simplest member of the graphite precursor series, yields an uncomplicated spectrum which reduces to one broad band peaking at 4430 ± 20 Å at low Ca concentrations. This band has a total width at half peak intensity of ≅ 250